1. Field of the Invention
This invention relates generally to passive optical networks or PONs and more particularly to transmitter/receiver photonic integrated circuits (PICs) for optical line terminals (OLTs) or optical network units (ONUs) in passive optical network (PONs).
2. Description of the Related Art
A still most common manner of communication with residential customers is common twisted-pair telephone lines used for telephony and internet access such as via DSL or dialup modem. Increasingly, there is a need for high bandwidth data links to residential as well as commercial customers and optical fiber distribution networks are the networks of choice. Extending communication fibers to these customer locations would relieve the bandwidth bottleneck but that ‘last mile’ is an expensive endeavor to extend communication fiber-to-the-home or FTTH, also referred to as a fiber-to-the-premise or FTTP. A major architecture for FTTH is the deployment of passive optical networks or PONs which provided optical feed from a central office or optical line terminal (OLT), and includes an optical transmitter/receiver or transceiver, to a number of optical network terminals (ONTs) that each include an optical transmitter/receiver or transceiver commonly referred to as an optical network unit (ONU). Such ONUs may include more than one such transmitter. The network between the an OLT and ONUs is nothing but optical communication fiber, optical splitters, waveguide routers and/or filters at intermediate network nodes of a PON—all optical passive components or elements and, hence, the designation, passive optical network or PON. PONs are distinguished from an active optical network (AON) in that the latter have active nodes that can include repeaters or optical to electrical to optical (OEO) converters that is accomplished with active or electrically power driven components or elements. In principal, there is no such power driven elements in a PON. Thus, PONs have been developed to eliminate the need of active elements and therefore provides a significant cost difference from the deployment of AONs. PONs are also called BIDI communication links or bidirectional optical links. Thus, the PON is designed principally for two-way or bidirectional communication between a central location or OLT and a number of subscribers having FTTP, each with an ONT including an ONU.
PONs can be configured in various examples of different PON architectural configurations are shown in FIGS. 1A-1D. These and other such PON configurations are shown in Chapter 10, Optical Access Networks, of the text book entitled, “Optical Fiber Telecommunications IVB Systems and Impairments”, edited by Ivan P. Kaminow et al., 2002. FIG. 1A illustrates a basic tree PON comprises an optical splitter, such as a 1×n coupler, for coupling the optical line or bus from the OLT to a plurality of ONUs. FIG. 1B illustrates a bus PON comprising a plurality of n 1×2 couplers coupled to the OLT bus and each respective coupler is coupled to an ONU. FIG. 1C is a trunk protected tree PON which comprises an OLT that has two units, one that is active and the other is on standby which is both optically coupled to via separate optical spans to an optical splitter, such as a 2×n coupler, for optical coupling both OLT lines to a plurality of ONUs. It can readily be seen that if there is a failure of communication with one OLT, the other OLT can automatically be activated to perform transmit and receive functions in the PON. FIG. 1D illustrates a fully redundant tree PON which comprises not only redundant OLTs but also redundant ONUs which have two units, here marked L and R, where the L-ONUs at each premise are coupled to a first 1×n coupler and corresponding or paired R-ONUs at each premise are coupled to a second 1×n coupler. The first and second couplers are, in turn, optically coupled, respectively, to an L-OLT and an R-OLT. In the case here, all premise subscribers are fully protected in receiving or transmitting communication with an OLT if either one of such paired units or terminals cease to operate. Lastly, FIG. 1E illustrates a fully redundant bus PON which comprises, again as in FIG. 1D, redundant L- and R-OLTs and L- and R-ONUs but only one optical bus or line is required from the OLTs as a fiber loop, rather than two separate buses or lines coupled to separate couplers, which, in turn, are then coupled to one of the paired ONUs. Thus, there are n 2×2 couplers in the fiber loop for each paired L and R ONUs at a premise. While redundancy may be considered extravagate for a PON designed to be the least expensive as possible, the costs of the OLTs and ONUs can be also be driven by low life expectancy so that inexpensive short life units or terminals may be designed and manufactured at lower costs to significantly offset costs attributed to the deployment in a PON of paired or two units at the premises or two terminals at the central office. It should be also noted that the photodetectors being fairly broadband, a wavelength-sensitive passive splitter can be deployed in PONs to direct traffic from one or more OLTs to several nodes, each node having a group of ONUs.
A typical OLT transceiver comprises a 1.5 μm transmitter using, for example, a 1490 nm direct modulated semiconductor laser and has a 1.3 μm receiver using, for example, a 1310 nm photodetector (such as a PIN or APD type), both of which are coupled to a WDM coupler to an input/output access to the PON. The OLT also includes a video amplifier for supplying gain to video signals which is WDM coupled to the input/output access point of the OLT. The typical ONT includes an input/output access to the PON with a WDM coupler coupled to a 1.5 μm receiver, using for example, a 1490 nm photodetector (such as a PIN or APD type) and a 1.3 μm transmitter using, for example, a 1310 nm direct modulated semiconductor laser (DML). The WDM coupler is also coupled to an RF amplifier to receive a video signal. The standard ONT may include many different subassemblies from different vendors, such as a laser module, a photodetector module, a TO-can package, a transimpedance amplifier (TIA) IC chip, a bidirectional optical subassembly or BOSA assembly, and a transceiver assembly along with a limiting amplifier chip and a laser driver chip.
This invention is directed to the design and manufacture of transceivers for OLTs or ONUs, particularly ONUs, which are monolithically integrated on a single semiconductor substrate. This is not to say that such monolithic ONUs are not already known in the art. For example, U.S. Pat. No. 5,796,883 discloses one type of approach for an optical integrated circuit or photonic integrated circuit (PIC) that may be employed as a transceiver in an ONU. The disclosed PIC chip comprises InP-based chip that includes, in monolithic form, a circuit input/output that may have spot size convert, coupled via a 3 dB coupler to both a receiver photodetector (R-PD) at 1.5 μm and to a DFB laser diode (LD) at 1.3 μm. Also, a 1.3 back facet monitoring photodetector (BF-MPD) is integrated with the LD to monitor its output power. Thus, this transceiver functions as an ONU by receiving, for example, 1550 nm optical signals for conversion into electrical signals and transmitting 1310 optical signals for communication with an OLT in a PON. In further embodiments of the disclosure, the chip may be designed to receive both 1310 nm and 1550 nm signals separated via a directional coupler as well as transmit a 1310 nm signal through the same coupler. In this case, it is further suggested that the R-PD and the BF-MPD have an active region bandgap a little larger than the received 1.3 μm signal and the 1.3 μm LD active region bandgap, such as a bandgap at 1.35 μm, so as to improve the absorption efficiency of 1.3 μm light in either photodetector and, therefore, improve their performance.
U.S. Pat. No. 5,031,188 to Koch illustrates another type of approach for a PIC chip that may also be employed as in a transceiver in an ONU. In the approach here, there is no coupler at the chip input but rather a single longitudinal 1.3 μm bandgap waveguide which waveguide forms an inline integrated element train comprising at the chip output a 1.3 μm bandgap DFB or DBR LD at the chip input and used for generating and transmitting a direct modulated 1310 nm signal to the chip output, an intermediate positioned absorber to absorb 1310 nm back reflections from the LD so as not to be detected by a backend photodetector, and a backend receiver photodetector (R-PD) for receiving and detecting a modulated 1550 nm signal received at the chip input. Also, further embodiments utilize an electrical isolation region between the absorber and the R-PD so that there is no electrical parasitic interference between the LD and R-PD. Since the received 1550 nm signal is a longer wavelength than 1.3 μm bandgap waveguide that includes the initial LD in the train of elements and produces a signal wavelength at 1310 nm, the longer wavelength 1550 nm signal will not be generally absorbed by either the LD or absorber but will be absorbed at the negatively biased backend R-PD. Therefore, all these elements can be integrated within an inline waveguide as illustrated in the embodiments of the patent.
A companion patent to the '188 patent, U.S. Pat. No. 5,144,637 to Koch discloses a transceiver that may be utilized in the OLT. In this disclosure compared to the '188 patent disclosure, the inline waveguide is still a 1.3 μm bandgap layer but the LD includes a coupled 1.5 μm bandgap active region evanescently coupled to the inline waveguide and, further, the inline positions of the R-PD and the 1.5 μm LD are reversed. In this way, an incoming modulated 1.3 μm wavelength is immediately detected at the initial R-PD element at the chip input/output and a modulated 1.5 μm wavelength signal from the direct modulated backend LD is transparent to the inline waveguide and the 1.5 μm signal is transmitted out of the input/output of the chip. Also, disclosed are means for electrical isolation between the R-PD and the LD for the reasons previously stated as well the additional deployment of a SOA between the isolation region and the LD to enhance the gain of the direct modulated and transmitted 1550 nm signal.